ION KINETIC-ENERGY DISTRIBUTIONS IN ARGON RF GLOW-DISCHARGES

Kinetic-energy distributions have been measured for different mass-selected ions sampled from 13.56 MHz rf glow discharges in argon inside a "GEC rf reference cell." The electrode geometry of this cell produces in asymmetric discharge and the cell is operated in a pressure regime where ion-molecule collisions in the sheath region of the discharge are significant. Ions are sampled from the side of the plasma perpendicular to the interelectrode axis using an electrostatic energy analyzer coupled to a quadrupole mass spectrometer. Kinetic-energy distributions for Ar+, Ar2+, Ar++, and ArH+ are presented as functions of applied rf voltage, gas pressure, and distance of the mass spectrometer entrance aperture from the edge of the electrodes. The distributions obtained for the sampling orifice placed close enough to the electrodes to allow formation of a sheath in front of the orifice exhibit features similar to those observed previously when sampling ions through the grounded electrode of a parallel-plate reactor. The Ar+ and Ar++ distributions exhibit secondary maxima predicted to result from the formation of low-energy (thermal) ions in the sheath region, such as by charge-exchange and high-energy electron collisions. Kinetic-energy distributions for Ar2+ and ArH+ exhibit no secondary maxima and are peaked at high energies indicative of the sheath potential, and consistent with a formation mechanism involving relatively low-energy collisions in the bulk plasma (glow region).